The long-term goal of this research is to develop electrical stimulation systems that provide home and workplace mobility to paraplegic and hemiplegic patients having paralysis arising from spinal cord injury, stroke or head injury. This work is directed toward the development of rehabilitative devices that do not require substantial orthoses. There is a need for improved control of motion, so as to achieve better balance, smoother and more energy efficient locomotion, reliable accomplishment of stair climbing and descent, and crutch walking. This project will addresses critical control problems whose solution is necessary for neuroprostheses to attain these goals. The underlying scientific hypothesis of the proposed work is that specified motions of the knee, ankle and hip can be achieved by feedback controllers which, on the basis of real time measurements of joint angles, foot endpoint position and foot contact forces, modulate the co-stimulation of muscles, and consequently the net moments and mechanical impedances at each joint, so as to obtain the desired motion. A second hypothesis is that there are multiple sets of stimulus waveforms that achieve the same motion, and that the amount of co-stimulation involved in each set is related to the ability of the control system to achieve the prescribed trajectory despite mechanical disturbances. A third hypothesis is that the amount of co-stimulation is related to the duration of effective control (e.g., the fatigue resistance). This project will test these hypotheses by developing and characterizing the performance of such feedback controllers. Specific Aims are: 1) to develop and experimentally verify predictive models of electrically-stimulated muscle response. Models will be developed for the ankle, knee and hip joints (separately and in combination), and will account for co-stimulation of agonist muscle electrodes, modulation of stimulus pulse frequency and width, passive and external mechanical loads, and muscle fatigue. Models relating electrical stimulation and joint angles to generated net joint moments and the active stiffness and damping of the resulting biomechanical systems will be considered; 2) to develop and evaluate the ability of feedback controllers that modulate electrical stimulation to achieve specified trajectories of joint angles and/or contact forces; and 3) the best of these controllers will be applied to the achievement of improved stair ascent and descent by paraplegic patients, through computer-controlled electrical stimulation.